1. Field of the Invention
This invention relates to cleanout tools for oil and gas wells, and, more particularly, to an improved cleanout tool and method for removing debris from a well having a low fluid level, the cleanout tool having only one service break while being simpler to assemble and operate in addition to providing for a more efficient cleaning of the well.
2. The Prior Art
An oil or gas well is a borehole drilled deep into the earth until it penetrates through the particular formation or formations from which the hydrocarbons of interest are to be extracted. The well is drilled into the earth at depths generally between about 1000 meters and 6,000 meters. Customarily, drilling to these depths is accomplished with a rotary drill system wherein a drill bit is mounted on the bottom of a hollow, rotatable drill stem, the drill stem being assembled from a plurality of lengths of drill pipe. The lengths of drill pipe are progressively added to the upper end of the drill stem as drill bit progresses downwardly through the various types of rock to create the borehole. A drilling fluid is forced downwardly through the hollow drill stem and is ejected a jets at the drill bit to lubricate the drill bit and to remove the cuttings away from the drill bit and to carry them to the surface. The drilling fluid is recovered, processed, and recycled through the drill stem.
As the drilling progresses downwardly, a steel casing is inserted as a liner into the borehole. This liner provides the necessary dimensional integrity for the borehole by supporting the surrounding earthen/rock surface as well as the conduit through which the drilling fluid/cuttings flow upwardly through the annular space surrounding the drill stem. Ultimately, the casing carries the hydrocarbon products from the well.
After the borehole has been drilled to the desired depth in the hydrocarbon-producing formation and the casing has been extended to the bottom of the borehole, the casing is perforated. Perforation is formed as a plurality of holes through the casing along a selected length of the casing. This length is calculated to generally correspond to the vertical thickness of the producing formation. The hydrocarbon products from the formation enter the well through these perforations and are subsequently recovered. This sequence of events is referred to in the art as production. During production, cuttings and sand from the formation pass through the perforations into the well. Further, rust and scale concretions form on and in the casing as a natural consequence of the refractory environment and the chemicals found in this region.
From time to time it is necessary also to treat the formation in order to stimulate production. Acid treatment is one technique for stimulating production. During acid treatment a propping agent such as a specially selected sand is pumped into the well either during or after the acid treatment operation. This procedure is known as acid fracturing and results in a substantial quantity of surplus sand filling the bottom portion of the hole to a depth of thirty or more meters. This sand residue along with the aforementioned cuttings, scale, and the like, must be removed from the well before production can be resumed. In addition to the foregoing debris, elastomeric plugs referred to as frac balls are used to plug selected portions of the casing perforations during the acid fracturing sequence. These frac balls must also be removed.
One recent development is the well cleanout tool of Harrison (U.S. Pat. No. 4,190,113) which involves an elongated body having a pump means connected to a debris-retaining chamber. The pump means is actuated by reciprocating the tubing to cause fluid to move through the tool. Debris in the fluid settles out in the debris-retaining chamber while debris-free fluid flows through an outlet in the upper end of the tool and back into the annular space between the cleanout tool and the casing. Fluid in the casing is recirculated to carry additional debris into the debris-retaining chamber. A splined driveshaft enables the cleanout tool to be rotated while the pump is being reciprocated.
A combination cleanout and drilling tool is disclosed in the reference of Moody et al (U.S. Pat. No. 4,421,182) and is designed to permit cleanout of a borehole in either a hydrostatic or hydraulic operation. Additionally, the tool may be used to drill a formation within the borehole without the need for circulation of the drilling fluid to the surface to remove cuttings from the formation.
Various other types of sand pumps and bailers are shown in the patents of Palm (U.S. Pat. No. 563,055); Swan (U.S. Pat. No. 1,537,201); Gates (U.S. Pat. No. 2,000,750); and Dumble (U.S. Pat. No. 2,180,935).
Experience with these various types of cleanout tools has shown that they are susceptible to becoming fouled by scale and other debris during operation such that the effectiveness of the pumping action for drawing debris into the debris-retention chamber is substantially diminished. One primary cause of this loss of pumping efficiency is in the type, number, and placement of the one-way check valves required. For example, the valves shown in Harrison are extremely susceptible to debris-caused interference such that the valves rapidly lose efficiency. Also, the pump member is located toward the lower end of the cleanout tool where it is exposed to the highest concentration of debris in the incoming, debris-laden fluid so that it is subjected to an extremely high degree of interference with its pumping efficiency. Further, the check valve system of Moody is believed to be highly susceptible to plugging due to the inherent nature of the ball valve-type check valves.
Experience has also shown that these prior art devices do not function well under low fluid level conditions in the well. Low fluid levels are frequently encountered when the well is producing from what is referred to in the trade as a weak formation. A weak formation is one that allows fluid to pass into the formation so that only a relatively low hydrostatic head is possible in the well. This means that it is not possible to use the cleanout tool to pump the fluid out of the well, but the cleanout tool must be used to circulate the limited amount of fluid in the well while at the same time retaining debris in the cleanout tool for subsequent removal from the well.
In view of the foregoing, it would be a significant advancement in the art to provide a cleanout tool whereby the pumping member in the cleanout tool is located toward the upper end of the cleanout tool and at a position above the debris-retention chamber so as to better isolate the pumping member from interference by debris. It would also be an advancement in the art to provide a cleanout tool having novel check valves that are simple in construction and less prone to becoming jammed by debris. An even further advancement in the art would be to provide a cleanout tool that is easily disassembled for cleaning and repair. Such a novel apparatus and method is disclosed and claimed herein.